When high-carbon steels are prepared for cold deformation, they are first heat treated. Particularly, when high carbon steel wire is prepared for cold deformation, the heat treating operation, known as "patenting" is performed in a continuous line. That is, the wire is run off of spools or coils through the heat treating furnace in a continuous strand. Following the heat treating operation, the steel wire is provided with a coating to aid in the subsequent deformation. Coatings on the steel surface in conjunction with suitable drawing lubricants, are known to aid in reducing friction between the metallic surface and the deforming tools. Coatings formed on the surface of metals, which are subsequently to be cold deformed, are required to serve functions in addition to aiding in the cold deformation of the steel. When the metallic article is not immediately deformed, a coating on the metallic surface provides protection against surface corrosion. In addition, the coated work pieces when passed through drawing lubricants prior to deforming, retain a greater amount of the lubricants than do non-coated surfaces. Phosphate coatings improve wire drawing operations by increasing the drawing speed, providing longer die life, and improving machine efficiency due, for example, to less stoppage for die replacement. They also substantially reduce die maintenance cost due to a more regular wearing of the dies. Better appearance and surface condition of the drawn wire, greater uniformity of cross section and increased resistance to corrosion are other advantages available from using phosphate coatings. The ability to draw wire of higher carbon content and higher tensile strength at increased speed is a further advantage.
Among the many phosphate coating compositions which are available to form the coatings, a zinc phosphate coating is preferred as an aid in wire drawing. Iron and manganese phosphate coatings have been tried and found substantially inferior as drawing aids. In addition, the manganese phosphate coatings are slower reacting, more costly to use and more difficult to control. Iron phosphate coatings do not produce sufficient coating weights for their use in wire drawing.
The optimum amount of phosphate coating for continuous wire drawing involving several size reduction steps is one which will give good performance at the last die without being excessive at the first die. Phosphate coatings can be too heavy and result in the creation of vibrations during size reduction in the drawing steps. Too light a coating will result in poor performance at the last die because insufficient coating is carried through to maintain the required separating layer between the die and the wire to prevent excessive wear of the die or scratching of the workpiece surface.
Coating weights of from about 450 to 1,550 mg/sq. ft. (5 to 16 g/m.sup.2) of surface area are desirable for steel wire drawing. To obtain coating weights in excess of about 450 mg/sq. ft. (5 g/m.sup.2) zinc phosphate compositions have been employed.
In some zinc phosphate coating operations, additional ionic constituents have been employed. Additional ionic constituents such as nickel and copper have been thought to influence the reaction of coating compositions on the metal surface and modify the character and properties of the phosphate coatings.
Phosphate coating compositions having phosphate, zinc and nitrate have been employed in the past. In addition compositions wherein the nitrite oxidizing agent is autocatalytically regenerated from nitrate in an operating coating bath have been employed. The nitrate and nitrite are useful in the operating coating bath to convert the dissolved iron from the divalent ferrous form into the trivalent ferric form. Since tertiary ferric phosphate is only slightly soluble in an acid phosphate coating bath, the dissolved iron is precipitated out of the solution as sludge and the bath is thus maintained in a so-called "iron-free" condition.
In the attempts to form zinc phosphate coatings it has not been possible to form coatings of greater than about 450 mg/sq. ft. (5 g/m.sup.2) on high carbon steel in less than about 30 seconds. Additionally the autocatalytic regeneration of nitrite fails at the low bath loading rates common to the continuous strand processing methods.
Because zinc phosphate coatings of greater than about 450 mg/sq. ft. (5 g/m.sup.2) could not be formed with sufficient speed to be incorporated into a continuous strand, wire heat-treating line, the wire has frequently been recoiled subsequent to heat treating for a later coating operation. The recoiled wire could then be contacted with a coating bath over a longer period of time by slowing the speed of the moving wire. The recoiled wire has also been coated in the coiled or rolled state. When a coil or roll of wire is coated in the coil or roll, that is without unrolling, the resulting coating is not uniform in thickness over the entire surface area, because the coating composition is not contacted uniformly with the metal surface throughout the suspended coil or roll. Sophisticated apparatus must then be used to shake the coils and rolls or drop them sharply in attempts to open the coils or rolls enabling better contact with the coating baths. Because of the time and costs involved for recoiling and coating, it is desired to eliminate these steps. To eliminate the recoiling and separate treating steps, attempts to apply a conversion coating on the steel surface as the wire leaves the heat-treating step have been made.
When applied to high carbon steel wire "patenting" or heat-treating lines moving at high speed in a continuous operation, the prior art phosphate coating processes have been found deficient in their application. High carbon steel has been found to be particularly difficult to coat. To withstand the heavy pressures in the deforming operation, particularly in those of high carbon steel wire drawing, coatings of from about 450 to about 1,200 mg/sq. ft. (5 to about 13 g/m.sup.2) coating weight are desired. When small diameter high carbon steel wire is heat treated, speeds of up to 200 feet (70 meters) per minute have been obtained. Such high carbon steel wire in a continuous heat-treating operation, when immediately provided with a zinc phosphate coating without rerolling or recoiling subsequent to heat treating and prior to coating, has required a coating stage of up to about 100 feet (30.5 meters) due to the long coating time required. The costs of apparatus and work space make such a practice prohibitive. To save space, which is at a premium, it is desirable to limit the coating stage to no more than about 25 feet (7.6 meters).
Attempts to rapidly apply uniform zinc phosphate coatings on surfaces of high carbon steel wire have resulted in coatings which are deficient in coating weight.